There are a host of applications which use a solid state controlled variable speed drive to operate an electric motor under various torque and speed conditions. Generally, the power source for these drives is a constant voltage supply. A typical configuration is a direct current (DC) source connected to an array of solid state bridges which are, in turn, connected to an alternating current (AC) motor. The solid state switches typically use pulse width modulation (PWM) to control the polarity and magnitude of the drive current being delivered to the motor and in this way control drive torque and speed of the motor in a maximum operating mode when drive torque and speed of the motor is at a maximum and in a minimum operating mode when the torque and speed of the motor is at a minimum.
One problem with this approach is that the performance of the motor and associated drive control circuits exhibit switching losses and PWM losses in the motor windings at partial load and speed. One reason for these losses is that at lower speeds the motor impedance and effective back electromagnetic force (emf) are decreasing which puts increasing demands on the inverter which is operating predominantly in the buck mode. This condition at partial speeds is particularly severe in marine type applications where the load torque varies quadratically with the shaft speed of the motor. This situation is exacerbated by the fact that operation of many naval drives are predominantly at partial speed.
In addition as the motor size and output power requirements increase, the excitation voltage and/or current associated with a single set of motor windings may exceed the ratings of the solid state switches that drive the motor windings. One method of overcoming this problem is to combine multiple solid state switches in series and/or in parallel. Another common method is to divide the motor windings into a number of discrete windings such that the voltage and current excitation required to drive each discrete winding is matched with the rating of a single switch.